Polyethylene terephthalate (hereinafter, also referred to as PET) is excellent in transparency, mechanical performance, melt stability, solvent resistance, aroma retaining properties, and recycling efficiency. PET is therefore widely used as a material for films, sheets, hollow containers, and the like. Some of its properties, such as heat resistance, are however not sufficient. In this respect, attempts have been made, for example, to modify PET by copolymerization.
Examples of the modification by copolymerization include the modification of a polyester resin using a compound having a cyclic acetal skeleton. Specific examples of such a modified polyester resin include PET modified with a diol 3,9-bis(1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane (see e.g., Patent Literature 1). Another example thereof includes a polyester resin comprising terephthalic acid, 1,4-butanediol, and a diol having a cyclic acetal skeleton as monomers (see e.g., Patent Literature 2). Alternative examples thereof include a polyester resin comprising a diol having a cyclic acetal skeleton as a monomer (see e.g., Patent Literature 3).
Examples of cases using adhesion properties derived from an acetal bond include a polyester adhesive obtained from a diol having a cyclic acetal skeleton including 3,9-bis(1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane, and a dicarboxylic acid, and adhesive compositions and coating agents prepared by using the adhesive (see e.g., Patent Literatures 4, 5, 6, and 7). Examples of cases using a polyester resin obtained from a dicarboxylic acid and a diol having a cyclic acetal skeleton include polyester-based mixed filament yarns with different shrinkage, modified polyester films, biodegradable polyesters, toners for electrostatic development, and flame-retardant resin compositions (see e.g., Patent Literatures 8, 9, 10, 11, and 12).
Examples of general methods for producing polyester resins as described above include direct esterification and transesterification methods. The direct esterification method involves converting dicarboxylic acids to esters using diols in excessive amounts and polycondensing the esters under reduced pressure to form a polymer. The transesterification method involves transesterifying esters of dicarboxylic acids and alcohols, and diols in excessive amounts to form esters of dicarboxylic acids and diols and polycondensing the esters under reduced pressure to form a polymer.